Ubuntu Manpage: GNU-VPE - Overview of the GNU Virtual Private Ethernet suite.

NAME

       GNU-VPE - Overview of the GNU Virtual Private Ethernet suite.

DESCRIPTION

       GVPE is a suite designed to provide a virtual private network for multiple nodes over an
       untrusted network. This document first gives an introduction to VPNs in general and then
       describes the specific implementation of GVPE.

   WHAT IS A VPN?
       VPN is an acronym, it stands for:

       Virtual
           Virtual means that no physical network is created (of course), but a network is
           emulated by creating multiple tunnels between the member nodes by encapsulating and
           sending data over another transport network.

           Usually the emulated network is a normal IP or Ethernet, and the transport network is
           the Internet. However, using a VPN system like GVPE to connect nodes over other
           untrusted networks such as Wireless LAN is not uncommon.

       Private
           Private means that non-participating nodes cannot decode ("sniff)" nor inject
           ("spoof") packets. This means that nodes can be connected over untrusted networks such
           as the public Internet without fear of being eavesdropped while at the same time being
           able to trust data sent by other nodes.

           In the case of GVPE, even participating nodes cannot sniff packets send to other nodes
           or spoof packets as if sent from other nodes, so communications between any two nodes
           is private to those two nodes.

       Network
           Network means that more than two parties can participate in the network, so for
           instance it's possible to connect multiple branches of a company into a single
           network. Many so-called "VPN" solutions only create point-to-point tunnels, which in
           turn can be used to build larger networks.

           GVPE provides a true multi-point network in which any number of nodes (at least a few
           dozen in practise, the theoretical limit is 4095 nodes) can participate.

   GVPE DESIGN GOALS
       SIMPLE DESIGN
           Cipher, HMAC algorithms and other key parameters must be selected at compile time -
           this makes it possible to only link in algorithms you actually need. It also makes the
           crypto part of the source very transparent and easy to inspect, and last not least
           this makes it possible to hardcode the layout of all packets into the binary. GVPE
           goes a step further and internally reserves blocks of the same length for all packets,
           which virtually removes all possibilities of buffer overflows, as there is only a
           single type of buffer and it's always of fixed length.

       EASY TO SETUP
           A few lines of config (the config file is shared unmodified between all hosts) and
           generating an RSA key-pair on each node suffices to make it work.

       MAC-BASED SECURITY
           Since every host has it's own private key, other hosts cannot spoof traffic from this
           host. That makes it possible to filter packet by MAC address, e.g. to ensure that
           packets from a specific IP address come, in fact, from a specific host that is
           associated with that IP and not from another host.

PROGRAMS

       Gvpe comes with two programs: one daemon (gvpe) and one control program (gvpectrl).

       gvpectrl
           This program is used to generate the keys, check and give an overview of of the
           configuration and to control the daemon (restarting etc.).

       gvpe
           This is the daemon used to establish and maintain connections to the other network
           nodes. It should be run on the gateway of each VPN subnet.

COMPILETIME CONFIGURATION

Please have a look at the gvpe.osdep(5) manpage for platform-specific information.

Gvpe hardcodes most encryption parameters. While this reduces flexibility, it makes the
program much simpler and helps making buffer overflows impossible under most
circumstances.

Here are a few recipes for compiling your gvpe, showing the extremes (fast, small,
insecure OR slow, large, more secure), between which you should choose:

AS LOW PACKET OVERHEAD AS POSSIBLE
./configure --enable-hmac-length=4 --enable-rand-length=0

Minimize the header overhead of VPN packets (the above will result in only 4 bytes of
overhead over the raw ethernet frame). This is a insecure configuration because a HMAC
length of 4 makes collision attacks almost trivial.

MINIMIZE CPU TIME REQUIRED
./configure --enable-cipher=bf --enable-digest=md4

Use the fastest cipher and digest algorithms currently available in gvpe. MD4 has been
broken and is quite insecure, though, so using another digest algorithm is recommended.

MAXIMIZE SECURITY
./configure --enable-hmac-length=16 --enable-rand-length=12 --enable-digest=ripemd610

This uses a 16 byte HMAC checksum to authenticate packets (I guess 8-12 would also be
pretty secure ;) and will additionally prefix each packet with 12 bytes of random data.

In general, remember that AES-128 seems to be as secure but faster than AES-192 or
AES-256, more randomness helps against sniffing and a longer HMAC helps against spoofing.
MD4 is a fast digest, SHA1, RIPEMD160, SHA256 are consecutively better, and Blowfish is a
fast cipher (and also quite secure).

HOW TO SET UP A SIMPLE VPN

In this section I will describe how to get a simple VPN consisting of three hosts up and
running.

STEP 1: configuration
First you have to create a daemon configuration file and put it into the configuration
directory. This is usually /etc/gvpe, depending on how you configured gvpe, and can be
overwritten using the -c command line switch.

Put the following lines into /etc/gvpe/gvpe.conf:

udp-port = 50000 # the external port to listen on (configure your firewall)
mtu = 1400 # minimum MTU of all outgoing interfaces on all hosts
ifname = vpn0 # the local network device name

node = first # just a nickname
hostname = first.example.net # the DNS name or IP address of the host

node = second
hostname = 133.55.82.9

node = third
hostname = third.example.net

The only other file necessary is the if-up script that initializes the virtual ethernet
interface on the local host. Put the following lines into /etc/gvpe/if-up and make it
executable (chmod 755 /etc/gvpe/if-up):

#!/bin/sh
ip link set $IFNAME address $MAC mtu $MTU up
[ $NODENAME = first ] && ip addr add 10.0.1.1 dev $IFNAME
[ $NODENAME = second ] && ip addr add 10.0.2.1 dev $IFNAME
[ $NODENAME = third ] && ip addr add 10.0.3.1 dev $IFNAME
ip route add 10.0.0.0/16 dev $IFNAME

This script will give each node a different IP address in the 10.0/16 network. The
internal network (if gvpe runs on a router) should then be set to a subset of that
network, e.g. 10.0.1.0/24 on node first, 10.0.2.0/24 on node second, and so on.

By enabling routing on the gateway host that runs gvpe all nodes will be able to reach the
other nodes. You can, of course, also use proxy ARP or other means of pseudo-bridging, or
(best) full routing - the choice is yours.

STEP 2: create the RSA key pair for each node
Next you have to generate the RSA keys for the nodes. While you can set up GVPE so you can
generate all keys on a single host and centrally distribute all keys, it is safer to
generate the key for each node on the node, so that the secret/private key does not have
to be copied over the network.

To do so, run the following command to generate a key pair:

gvpectrl -c /etc/gvpe -g nodekey

This will create two files, nodekey and nodekey.privkey. The former should be copied to
/etc/gvpe/pubkey/nodename on the host where your config file is (you will have to create
the pubkey directory first):

scp nodekey confighost:/etc/gvpe/pubkey/nodename

The private key nodekey.privkey should be moved to /etc/gvpe/hostkey:

mkdir -p /etc/gvpe
mv nodekey.privkey /etc/gvpe/hostkey

STEP 3: distribute the config files to all nodes
Now distribute the config files and public keys to the other nodes.

The example uses rsync-over-ssh to copy the config file and all the public keys:

rsync -avzessh /etc/gvpe first.example.net:/etc/. --exclude hostkey
rsync -avzessh /etc/gvpe 133.55.82.9:/etc/. --exclude hostkey
rsync -avzessh /etc/gvpe third.example.net:/etc/. --exclude hostkey

You should now check the configuration by issuing the command gvpectrl -c /etc/gvpe -s on
each node and verify it's output.

STEP 4: starting gvpe
You should then start gvpe on each node by issuing a command like:

gvpe -D -l info first # first is the nodename

This will make the gvpe daemon stay in foreground. You should then see "connection
established" messages. If you don't see them check your firewall and routing (use tcpdump
;).

If this works you should check your networking setup by pinging various endpoints.

To make gvpe run more permanently you can either run it as a daemon (by starting it
without the -D switch), or, much better, from your inittab or equivalent. I use a line
like this on all my systems:

t1:2345:respawn:/opt/gvpe/sbin/gvpe -D -L first >/dev/null 2>&1

STEP 5: enjoy
... and play around. Sending a -HUP (gvpectrl -kHUP) to the daemon will make it try to
connect to all other nodes again. If you run it from inittab gvpectrl -k (or simply
killall gvpe) will kill the daemon, start it again, making it read it's configuration
files again.

To run the GVPE daemon permanently from your SysV init, you can add it to your inittab,
e.g.:

t1:2345:respawn:/bin/sh -c "exec nice -n-20 /path/to/gvpe -D node >/var/log/gvpe.log 2>&1"

For systems using systemd, you can use a unit file similar to this one:

[Unit]
Description=gvpe
After=network.target
Before=remote-fs.target

[Service]
ExecStart=/path/to/gvpe -D node
KillMode=process
Restart=always

[Install]
WantedBy=multi-user.target

AUTHOR

       Marc Lehmann <gvpe@schmorp.de>

COPYRIGHTS AND LICENSES

       GVPE itself is distributed under the GENERAL PUBLIC LICENSE (see the file COPYING that
       should be part of your distribution).

       In some configurations it uses modified versions of the tinc vpn suite, which is also
       available under the GENERAL PUBLIC LICENSE.